1. Field of the Invention
The present invention relates to a reversible thermosensitive recording material capable of recording and erasing images repeatedly by utilizing the property of reversibly changing the transparency from a transparent state to an opaque state, and vice versa, depending upon the temperature thereof.
2. Discussion of Background
Recently attention has been paid to a reversible thermosensitive recording material capable of temporarily recording images thereon and erasing the same therefrom when such images become unnecessary. As representative examples of this kind of reversible thermosensitive recording material, there are conventionally known reversible thermosensitive recording materials in which an organic low-molecular-weight material such as a higher fatty acid is dispersed in a matrix resin such as vinyl chloride-vinyl acetate copolymer with a glass transition temperature (Tg) of as low as 50.degree. C. or more to less than 80.degree. C., as disclosed in Japanese Laid-Open Patent Applications 54-119377 and 55-154198.
The above-mentioned conventional reversible thermosensitive recording materials have the defect that the recording materials cause a curling problem. More specifically, upon application of heat to the recording material, the material will curl with the thermosensitive recording layer on the inside of the curve while the image formation and erasure are repeatedly carried out by use of a heating means such as a thermal head.
In order to eliminate the above defect, there is a proposal of the formation of a protective layer on the reversible thermosensitive recording layer. This kind of protective layer, which is designed to protect the thermosensitive recording layer from the above-mentioned thermal hysteresis, comprises a silicone rubber or silicone resin (disclosed in Japanese Laid-Open Patent Application 63-221087), a polysiloxane graft polymer (disclosed in Japanese Laid-Open Patent Application 63-317385), or an ultraviolet-curing resin or electron radiation curing resin (disclosed in Japanese Laid-Open Patent Application 2-566).
However, in the case where a thick protective layer is provided on the recording layer, the protective layer and the reversible thermosensitive recording layer tend to shrink because the crosslinking reaction takes place while a coated liquid for the protective layer is dried under the application of heat thereto. For example, even when a reversible thermosensitive recording material employs as a support film a polyethylene terephthalate film with a thickness of 150 .mu.m or more, the curling problem occurs.
Due to the above-mention ed curling problem, the reversible thermosensitive recording material causes a transporting failure in an information recording apparatus when applied to a magnetic card such as a prepaid card.
By the way, a variety of methods for displaying information on the surface of a card-type reversible thermosensitive recording material are disclosed. For example, as colored portion is provided behind a reversible thermosensitive recording material as described in Japanese Laid-Open Utility Model Application 2-3876, a light reflection layer is provided behind a reversible thermosensitive recording material to improve the image contrast as in Japanese Laid-Open Patent Application 64-14079, and a thin film is provided behind a recording material, with the refractive index of the thin film being different from that of the recording layer as in Japanese Laid-Open Patent Application 2-175280.
However, when the above-mentioned colored portion is directly or indirectly applied to the thermosensitive recording layer, the image contrast obtained on the recording material is low. In addition, the image contrast cannot be sufficiently improved for practical use even by the provision of the above-mentioned light reflection layer or thin film.
In the case where heat energy is applied to the reversible thermosensitive recording material by using a heat-application roller or a heat-pen, with the application of slight pressure thereto, in order to perform recording and erasing operations, the durability of the recording material is not a problem even though the image formation and erasure are repeated. However, when both heat and pressure are applied to the recording material at the same time, for instance, by using a thermal head, the durability of the recording material becomes a problem during the repeated image formation and erasing operations. This is because the matrix resin has poor heat resistance. More specifically, the matrix resin around the particles of the organic low-molecular-weight material in the recording layer is deformed and the particle size of the organic low-molecular-weight material dispersed in the matrix resin is increased during the course of the repeated recording and erasing operations. As a result, the light scattering effect of the recording layer is decreased. As a matter of course, the degree of whiteness of a white opaque portion in the recording layer is also significantly decreased. In the end, the image contrast is disadvantageously lowered.
Further, the thermosensitivity of the conventional recording materials which are reversibly changeable between a transparent state and a milky white opaque state depending upon the temperature thereof is not sufficient for forming an image thereon by the application of a small amount of thermal energy from a thermal head.